A mixer truck is a vehicle that carries fresh concrete such as mortar or ready mixed concrete in a mixer drum mounted on a frame to be free to rotate, and transports the fresh concrete from a fresh concrete factory to a construction site.
The mixer truck prevents the fresh concrete from deteriorating in quality and hardening by rotating the mixer drum in a positive direction while transporting the fresh concrete and stirring the fresh concrete using a plurality of helical blades disposed in the mixer drum. Further, the mixer truck is configured to be capable of discharging the fresh concrete in the mixer drum by rotating the mixer drum in an opposite direction to the positive direction. Upon arrival at a concrete placement site, the mixer truck supplies the fresh concrete to a placement location by rotating the mixer drum in reverse.
In this type of mixer truck, the mixer drum must be rotated constantly until the fresh concrete is discharged. An engine of the mixer truck is typically used as a drive source of the mixer drum. More specifically, power from the engine is transmitted to a hydraulic pump via a PTO (Power Take Off), working oil discharged from the hydraulic pump is supplied to a hydraulic motor, and the mixer drum is driven to rotate by rotation of the hydraulic motor, which is driven by the working oil.
In a mixer drum driving device that drives the mixer drum using the engine alone, an engine rotation speed must be increased in order to rotate the mixer drum at a high speed or the like. When the engine rotation speed is increased, noise is generated and fuel consumption increases.
Further, while the fresh concrete is carried in the mixer drum, the mixer drum must be rotated continuously to prevent hardening and the like, and therefore the engine cannot be stopped. Hence, the engine must be driven continuously even when the mixer truck remains stationary while waiting in line to discharge the fresh concrete at the placement site.
JP2007-278430A and JP2003-301802A disclose mixer drum driving devices that drive a mixer drum to rotate by driving a secondary hydraulic pump using a motor as well as driving a main hydraulic pump using an engine.